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How many times has this happened to you?  It's a pleasant summer afternoon, so you pour yourself a glass of wine and take it outside to enjoy on your deck or patio.  Maybe you have a magazine, or are engaged in conversation, and after only a minute or two you reach for your glass for another sip and, from nowhere, you discover a couple of fruit flies doing the breast stroke in your wine.  The Spaniards, after deciding that enough is enough, even created a culinary solution to this annoying problem:  Tapas.

Those fruit flies weren't just lurking in your backyard, patiently waiting for your arrival, wine glass in hand. There is a strong possibility that they smelled the nose of that pinot noir from as far away as 3/4 of a mile and, like Homer Simpson, sighed to themselves "mmmmm, red wine."  They then high tailed it in your direction.  Tiny as they are, a fruit fly can travel as much as 6.5 miles a day in search of food.  Guided by scent, they follow their "noses", so to speak.

Annoying as they are, they are fascinating little critters.  I was given to thinking about them lately because, while they are supposed to be unable to survive temperatures approaching freezing, I still see them in the house, and if I take a glass of wine outside with me, even now, they still manage to find it with amazing speed.

I did a little sleuthing on the subject, so you wouldn't have to, and here's what I discovered.

fruit flies

The common fruit fly, Drosophila  melanogaster, is amazingly prolific.  With an average life span of about 30 days, its life cycle is also amazingly condensed.  An adult female can lay upwards of 500 eggs during her brief life.  Those eggs can hatch into larvae in as little as 12 hours under optimal conditions.  After the larvae feed for 4 days they pupate, and after another 3 or 4 days they emerge as adult fruit flies, ready to begin mating again within just a few hours.  Wash, rinse and repeat.

The rapidity with which fruit flies would suddenly appear around over ripe fruit was one of the observations, in the era before insect life cycles were understood, that led to the theory of "spontaneous generation", that life could arise from inorganic components.

One mathematician calculated that, given their size (< .25 inches), the number of eggs laid by one female, and their life span, a single pair of fruit flies could, over the span of one year, produce a mass of offspring that, were they all to live that long, would form a sphere whose diameter would fill the space between the Earth and the Sun.  Think about that.

Sort of puts the "World's Largest Ball of Twine", a source of inestimable civic pride and joy to the residents of Cawker City, Kansas, to shame, doesn't it?  
World's Largest Ball of Twine

The "fruitfulness" of Drosophila m., if you'll pardon the pun, and the short time it takes to breed a new generation of adults, has made them invaluable in the field of genetic research.  Though the groundbreaking research in genetics was done by Gregor Mendel in the 1860's, using pea plants, his work was not widely appreciated and understood until after his death in 1884.  The term 'genetics" wasn't even coined until 1905.  It was Thomas Hunt Morgan, in 1911, who first began using fruit flies to study genetic traits, and scientists began to understand the role of chromosomes in the transmission of physical traits.  The genome of Drosophila m. was mapped in its entirety in 2000, and to this day it is one of the most widely studied organisms by geneticists and medical researchers.

For you and I, however, the fruit fly's prolific nature and the rapidity with which their eggs hatch, makes them more of an annoyance than anything.  Once you notice you have a fruit fly problem around the house, they can be devilishly difficult to get rid of.  In the time it takes to get up and tenaciously pursue a couple of errant fruit flies in your house that have pestered you to your wit's end, another dozen could have, quite literally, hatched somewhere else unbeknownst to you.  It can feel like digging in sand.  It's not just the peaches in your fruit bowl that sustain them, it can be the film of scum you cannot see in some sink drain, a damp, sour mop in the garage, the water under your refrigerator that accumulates from condensation and harbors decaying organic matter from something you may have dropped and which splattered beneath the appliance where your mop couldn't reach, a potato or onion gone bad in your pantry.

Pass the wine, please...but I'll pass on the beets
It seems fruit flies and humans share the same affinity for the byproduct of fermentation...alcohol.  For fruit flies, this is most readily found on over ripe fruit.  When the skin of a ripe fruit becomes bruised or torn, wild yeast that naturally exist everywhere quickly attach themselves to the exposed sugars...fructose.  As they consume those sugars, fermentation takes place, albeit on a small scale.  Some of that sugar is converted to alcohol.  Fruit flies have incredibly well developed scent receptors that detect the volatile chemical compounds associated with this process.  They feed upon the surface of the fruit where this process is taking place, and that is where they lay their eggs.

A glass of wine is, especially red wine, which is more aromatic than white, is irresistible to them.  It's like crack cocaine for fruit flies.

In fact, scientists who study Drosophila have discovered that male fruit flies share a behavior in common with human males.  They often hit the bottle in the wake of multiple rejections from their female counterparts.  An article in the New York Times drew upon a study published in the journal Science which examined male fruit flies which were denied the opportunity to mate:

They were young males on the make, and they struck out not once, not twice, but a dozen times with a group of attractive females hovering nearby. So they did what so many men do after being repeatedly rejected: they got drunk, using alcohol as a balm for unfulfilled desire.

Fruit flies apparently self-medicate just like many humans do, drowning their sorrows or frustrations for some of the same reasons, scientists reported Thursday. Male flies subjected to what amounted to a long tease — in a glass tube, not a dance club — preferred food spiked with alcohol far more than male flies that were able to mate.  

At least they are incapable of posting compromising photos of their exes on the internet.

One thing you'll never see a fruit fly hovering over, though, is a beet.  There are many people who don't care for the earthy fragrance of beets...that scent that recalls the smell of freshly plowed fields.  It turns out that fruit flies are absolutely repelled by it.  That scent is the result of a single organic compound...Geosmin.  According to an NPR story, it is the same chemical that gives corked wine its characteristic smell.  Fruit flies assoce this chemical with rotten, dangerous "food", and avoid it completely.  In fact, their neurosystem has dedicated an entire neural pathway to detecting just this one chemical.  That's quite remarkable given how simple their neural system is.  They have set aside, so to speak, a valuable chunk of molecular real estate to detecting one single chemical.  The implications of this research have applications, however, in the development of future generations of pesticides that aren't based upon poisoning, but rather deterrence.  Researchers are looking into chemicals that simply make crops smell unattractive to pests.

I knew of the fruit fly's prominence in scientific research from my High School and college studies, but never gave much thought to them until 1975, when I was living in California.  Los Angeles discovered, for the first time, that the dreaded Mediterranean Fruit Fly, Medfly for short, had made an unwelcome appearance.  The Medfly is a completely separate species from Drosophilla m., whose scientific name is Ceratitus capitata.  As the common name suggests, it is also not native to North America.  It originated in North Africa, it is believed, and is a common and serious agricultural pest throughout the Mediterranean region.

Unlike our native fruit fly, which feeds upon and lays its eggs upon the outside of ripe fruit, and whose larvae feed upon the exterior of that fruit upon hatching, the Medfly lays its eggs underneath the skin of under ripe fruit.  When those eggs hatch, the larvae feed upon the interior flesh of the fruit as it ripens, and can turn the fruit into a mushy, inedible mess before they pupate and emerge as adults.  The Drosophila prefers soft fruit...peaches, apricots, pears, grapes, strawberries and such.  The Medfly will attack more than 250 varieties of fruit and vegetables, hard and soft, rendering them unsalable.

California's Governor at the time, Jerry Brown, declared an agricultural state of emergency, and implemented aggressive (and controversial) control methods.  Dispatching a veritable army of agricultural department foot soldiers throughout LA's residential neighborhoods and elsewhere, pheromone traps were placed in trees and monitored to map the pest's presence and advance.  600 million sterile male Medflies were released.  A massive program of ground spraying with the insecticide Malathion was undertaken, which many people were alarmed by.  Nevertheless, the program went forward, and within a year victory was declared (at a cost of over $1 million to taxpayers).  The Medfly subsequently reappeared in 1980, and Jerry Brown tried to rely upon the same measures that were used the first time around.  They proved insufficient to eradicate the pest and, reluctantly, but under serious pressure from both California's ag industry and federal USDA officials, he instituted the even more controversial measure of ariel spraying of Malathion.  Again, the threat was beaten back.

The Medfly continues to be pose a serious threat to fruit growers in Florida, Texas and California, and it's hard to say how much money has been spent monitoring and controlling the pest over the decades since it was first discovered.  Those efforts, however, are a perfect example of what the economist John Kenneth Galbraith termed "the privatization of profits and the socialization of costs", which is the hallmark of modern capitalism.  

If you find yourself beset by fruit flies, I don't know what to tell you.  Don't allow your fruit to over ripen.  Keep it in the fridge if possible.  You can try washing your fruit when you bring it home from the store, but really most supermarket fruit is harvested well before it is ripe, and not likely to have fruit fly eggs on it.  Some people believe that bananas are major culprits, but the literature is unclear.  They, too, are harvested while quite green, but more than most fruit you can find ripe bananas in the stores.  The fact that they are attracted to wine makes wine traps a simple and effective way of capturing and killing them.  You can also use apple cider vinegar if you are loath to share your cabernet sauvignon with the little buggers.  Simply leave a glass out with an inch of wine in it, and let their natural impulses do the rest.  They will happily drown to death.  

In the end, it's a lot easier than trying to swat them.

Originally posted to Keith930 on Sat Jan 05, 2013 at 11:37 AM PST.

Also republished by Backyard Science.

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